This section provides background information related to the present disclosure which is not necessarily prior art.
Larger batteries are composed of individual cells. Typically, a battery for hybrid or electric vehicles, respectively, or for industrial applications contains between twenty and several hundreds of individual cells. The individual cells can thereby be embodied as round cells, as prismatic cells or as so-called coffee bag cells. Coffee bag cells comprise a flexible shell, composed of a film, in which electric components of a cell are arranged.
To realize an optimal use of space in a battery, coffee bag cells are used above all. They are furthermore characterized by having a low weight, yet a high capacity. Coffee bag cells can be cooled well via the thermally conductive films. Cells of this design can furthermore be scaled easily, because the size of all of the components of the cell, including the film housing, can easily be varied in production.
Due to high stored energy quantities, larger batteries always represent a safety risk in response to the occurrence of malfunctions. Lithium batteries must thereby be considered to be particularly critical, because they encompass a high energy density, a combustible electrolyte and thin separators. Finally, lithium batteries generate high cell voltages, so that the components arranged in the cell are subjected to high electrochemical stresses. This is particularly relevant in the case of automobile and industrial batteries, for which life cycles of at least 8-10 years are assessed.
The afore-mentioned coffee bag cells can be installed in a space-saving manner. For example, large quantities of energy per unit of volume can be stored in a battery. However, considerable construction-related disadvantages are connected therewith. Due to the flexible shell, the dimension of coffee bag cells changes when they are charged or discharged. This is also connected to a volume expansion. The volume expansion leads to typical changes in thickness of an individual cell of approximately 5% between charged and non-charged state.
In the case of an assembly of a so-called “stack”, which consists of many individual cells, which are connected in series, it must thus be considered that the individual cells display a variable volume. In particular, it must be ensured that the cells in the charged state, in which they take on their largest thickness, apply virtually no pressure or only a slight pressure on the surfaces of adjacent cells. On principle, it must thereby also be considered that, due to the production tolerances, the thickness of the flexible cells is also not consistent, but is subject to fluctuations.
There is furthermore a demand for an arrangement, by means of which impacts or vibrations are absorbed and/or damped, so that the interior of the battery as well as contacts are not damaged. Connections of the conduction and control electronics must furthermore be connected to the battery so as to be as free from mechanical stresses as possible. A disconnection of even one of the many hundreds of contacts of the power electronics leads to a malfunction of the battery in response to a series connection. In the case of a malfunction of a contact of the control electronics, the cell, which is then no longer controlled, can gradually reach a critical state, which in the medium term can lead to a deterioration or breakdown of the entire battery.
The flanges of the afore-mentioned coffee bag cells encompass a sealed seam. This sealed seam connects two films of a cell, which thus enclose further components in the hollow space formed thereby. For this purpose, the inside of the films are coated with an electrically insulating, adhesion-promoting sealing thermoplastic. This sealing thermoplastic can be formed from a functionalized polyolefin. This sealed seam represents a mechanical weak spot of a coffee bag cell.
Furthermore, the air pressure in the environment of the cell can fluctuate. In the event that the housing of a battery is closed in an air-tight manner, temperature-related pressure fluctuations of typically 0.2 bar can occur. These pressure fluctuations additionally stress the sealed seams.
The sealed seam, however, also represents a predetermined breaking point, which is to allow the electrolyte to discharge in the event of a breakdown of the battery. Through this, a bursting of the cell is to be avoided. In the event that the escaping combustible electrolyte comes into contact with electrodes, it can ignite and can lead to fires or explosions. For the most part, the maximally permissible overpressures in the interior of a coffee bag cell are far below 0.1 MPa, so as to prevent an opening of the sealed seam. The lead-through of the power-diverting electrodes is to be considered to be particularly critical in the case of coffee bag cells. For the most part, they encompass a thickness of approximately 0.1 to 0.3 mm. In this range, a possible leakage is also particularly critical, because escaping electrolyte can instantaneously ignite at the electrodes. The sealed seam is generally considered to be a weak spot of large cells, because it is subjected to constant stresses for many years, which are caused by the cyclizing.
Finally, water-based cooling agents are currently preferably used for the cooling of large batteries or fluorinated carbon hydride or carbon dioxide in response to the use of air conditioning systems. A direct contact of most of the cooling agents with the interior of the cells can lead to a severe chemical reaction. In the case of water-based cooling agents, hydrogen, for example, which is highly flammable and which can lead to explosions, is released. A contact cooling is thus typically used in the art for this reason, wherein the thermal flow between cell and cooling circuit is established via thermally conductive components and the cooling agent can thus not be in direct contact to the cells.